Electropolishing stainless steel



NOV- 19, 1946. G. J. BEcKwlTH ELECTROPOLISHING STAINLESS STEEL Filed June 14, 1941 atented Nov. 19, 1946 ELECTROPOLISHING STAINLESS STEEL Glenwood J. Beckwith, Brecksville, hio, asslgnor to The American Steel and Wire Company of New Jersey, a corporation of New Jersey Application June 14, 1941, Serial No. 398,159

` 3 Claims. (Cl. 21M-140) The demand for highly lustrous ferrous alloys di the class known as stainless steels is increasing daily,v owing to the increasingly widespread.

use of such materials as ornamental and sanitary finishes and fittings in the innumerable applications of stainless steel.

However, the production of the lustrous iinishes to meet popular demand is attended with Y difficulty.

At the time when stainless steel was becoming introduced on the market, a good metal surface, whitened by pickling was the only material that was available practicably. But as the use of such steels increased, there arose a demand for highly lustrous surfaces, which were applied for ornamental purposes and which were obtained by purely mechanical operations of grinding, polishing, and burnishing. t

The appearance of such lustrous surfaces for ornamental uses whetted further the demand therefor, both from ornamental and utilitarian viewpoints; and the mechanical procedures heretofore employed proved to be inadequate to Sup- `plythe demand. A

Consequently, investigators turned to chemical,

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and then to electrochemical means forproducing the desired surface finishes. Chemical methods, while producing finished surfaces satisfactory for many purposes, nevertheless were actually inferior to the finishes produced by mechanical means. Although the cause of such failure is not known deiinitely, it may lie in the selective action of the relatively weak solutions necessary to produce a, bright finish on inherent differences in the surface of the metal.

Consequently, considerable attention has been given to electrolytic processes forvproducing the requisite finish. Highly concentrated solutions in themselves have no ability to produce bright surfaces. However, when such solutions are electrolyzed, it has been found that finishes may be obtained that are highly improved over those produced by straightI chemical methods, and which allegedly are comparable to the finish produced by mechanical means.

In such electrolytic procedures, the stainless steel article is made an anode in an electrolyte which usually is a highly concentrated solution of sulphuric, or chromic, or phosphoric acid, or mixtures of sulphuric and phosphoric acids, sulphuric and citric acids, or sulphuric and hydrofluoric acids. A phosphoric acidl electrolyte has been considered to be satisfactory in the past, such electrolyte being made up of four parts of con- Z centrated orthophosphoric acidby volume to one part of water. Y

Now, the present invention is based upon the discovery of a procedure wherein there are produced on stainless steel articles, lustrous finishes vthat are very substantially superior even to those produced by the procedures outlined above.

The present invention is based upon the discovery of an inert, or inhibitor material, which, when introduced into the electrolyte, obviates pitting of the metal surface and also obviates any selective action of the electrolyte on any part of the metal surface.

In the operation of any electrolytic polishing procedures, however, certain factors always are present.

Thus, it is well known that electrolytes are more conductive at elevated temperatures than they are at room temperatures. Many polishing baths, however, must be operated at, or only slightly above, room temperature, in order to avoid an undulated or, Vin some cases, a pittedv surface.

Also, it is well known that certain organic materials serve to inhibit pitting by polarization phenomena, as is true, for example, in the chem ical pickling of steels. But such materials do not so operate in the presence of` an electrolyzing current, with the treated article as an anode in the bath.

In accordancewith the present invention, it is found that pyridine possesses excellent properties for attaining the desired results. Pyridine has been used previously in a straight chemical descaling acidlbath, but its behavoir in the electrolytic bath is diiferent from other inhibitors, it providing an excellent protection for articles being electropolished.

While the concentrations of such chemical inhibitors are low, if such concentrations of pyri- 1 dine are increased in an electropolishing solution, an immediate eiect is noted upon the electrolyte and the resultant polishing effect.

Thus, the movement of the gas bubbles from the article being treated is slowed down greatly, and there is noted a change in the surface tension of the electrolyte. A foamy blanket appears on the surface of the bath similar to that on a chemical pickling bath when a foamy inhibitor is added. The slowing up of the gases in the bath alleviates the characteristic acid spray from the surface thereof and the air-liquid, extra-bright"I meniscus pattern on partially submerged articles disappears.

3 Pyridine derivatives, as well as pyridine itself, produce similar results. i

In practice, .it is found that an optimum electrolyte and working characteristics which produce brilliant finishes on stainless steels are;

Bath composition by volume The above bath operates very successfully without Sulphuric acid, but in practice the presence of sulphuric acid is preferable, since it adds to the metallic brilliance of the iiniSh. Too much sulphuric acid, however, tends to overcome the nonpitting effect of the pyridine and disturbs the polarization equilibrium. When no Sulphuric acid at all is used in the bath, the surface is extremely smooth,but is found to have a bluish cast` especially on the rustless type of steel, which is a vstraight chrome alloying steel containing from approximately 12% to approximately 17% chromium. The electrolyte may contain at least 50% by volume of a phosphoric acid of 85% grade and approximately 5% by volume of pyridine, the balance being water, l

In operation, the temperature should be held substantially constant at a definite selected value, and the current density controlled accordingly, since a fluctuation of either produces a slightly different degree of polishing in a given time. The

foregoing conditions are found, however, to produce excellent polished surfaces and by proper manipulation and control, markedly superior results over raw acid electrolytes are obtained.

The time of the treatment obviously is dependent upon the amount of brilliance desired on the animo 22, 23 to the negative terminal 2t of the generator it, which for convenience is shown as being mounted on a shelf projecting from the cell.

The bus-bars form permanent connections' and are conveniently'located above the level of the electrolyte to avoid excessive action of electrolyte spray thereon.

In operating, it is highlyimportant that a uniform flow of current be obtained from all parts of the surface in order` to avoid shaded or undulated areas. Also, the hanger 6 is designed to assure an even distribution of current over the entire area of the anode 2 being polished. The hanger 6 is insulated in all parts exposed to the bath except on the area making intimate contact with the article being polished in order to prevent dissolution of the hanger and wastage of current.

- It will be understood, of course, that the invention is not limited to the use of a phosphoric acid bath, or a mixturey of phosphoric acidfand sulphuric acid, containing pyridine, as has been ex pressed preferentially herein, it having been found that the addition of pyridine to any of the electrolytes enumerated herein; both, to those nished article, and the 'condition of its initial surface. the conditions herein disclosed is very rapid.

The voltage is kept preferably within the limits shown, since higher voltages tend to overheat the solution due to a given input to secure agiven f current density, and higher voltages also tend to In the sing1e view of the drawing, the article 2 to be polishedis hung into the bath 4 on a hook or clamp 6 capable of carrying the current'required. The bath 4 is contained in a suitable con- A tainer 8 which may be composed of chemical brick,

or other suitable material.

The article 2 forms an anode and its suspending hanger 6 engages a copper bus-bar l0 which is connected by current lead Il to the positive terminal I2 of a suitable current generator I4.'

Cathodes I6 of suitable material are spaced properlyaround the article 2, these cathodes being connected through bus-bars I8 and leads 20,

In general, the polishing action under containing mineral acids alone and mixtures of mineral acids with organic acids (e. `g., citric acid), likewise has a beneficiating effect. It may be noted, however; that the preferred composition herein set forth produces the high polish on both stainless steels containing both chromium and nickel as well as straight chromium-containing steels, the latter usually requiring a little longer time under the conditions set forth above, for. example about four minutes for straight chrome-steels containing 12% and' 17% chromium as against two minutes for chrome-nickel steel containing 18% chromium and 8% nickel.

As has been mentioned previously herein, both pyridine and soluble pyridine derivatives may be employed. Among such may be mentioned specifically, conyrine, piperidine, or any other watersolublepyridine composition. Pyridine and these components of pyridinemay be referred to as pyridine-base compounds' which expression therefore is used generically in the claims.

`Likewise, there may be employed other soluble phosphate-ion producing materials in addition to orthophosphoric acid= Thus,; metaphosphoric acid, or pyrophosphoric acid may be employed.

.The process therefore is quite flexible in its production of its desirable results.

1. A methodv of anodically polishing stainless. steel, characterized by immerslng the said steel in an electrolyte consisting of approximately 5% by volume of pyridine, approximately by 80% by volume of phosphoric acid of 85% grade,

approximately 15% by volume of 60 `Bauxn sul phuric acid, and about 5% by volume o f a pyridine compound, providing suitable cathodes to assure uniform current flow from all parts of the steel anode, maintaining the electrolyte at temperatures between approximately F. to approximately 215 F.,

and passing a current 12 volts.

s through the resulting cell at a current density of substantially 5 amps. per square inch of anode surface and a voltage-ot from about 7 to about Brede. together with approximately 15%'bylvo1- -unie ot V60" Baume snlphuric acid, and about 5% 'by volume oi' a soluble pyridine compound,pro

viding suitable cathodes to assure uniform current flow from al1 parts of the steel anode, and passing a current through .the resulting cell of suilicient densityand for a sumcient length of time to effect a polishon the said steel surface. GLENWOOD J. BECKWI'I'H. 

